The present invention relates to refractory vessels and more particularly to a novel dome design for the refractory liner for such a vessel.
Black liquor is a by-product of the wood pulping process. Black liquor is a mixture of hydrocarbon, caustic, chlorine and other corrosive chemicals. It is normally completely combusted in a recovery boiler. Inorganic chemicals including sodium sulfate and sodium sulfide are recovered for reuse in the pulping process. Heat produced by the complete combustion is converted to steam, which in turn is used to produce process heat and/or electrical power. An alternative device proposed for recovering inorganic chemicals from black liquor is a gasifier. In a gasifier, the black liquor is burned in a sub stoichiometric atmosphere to produce a combustible gas. Inorganic salts are recovered in the process. The combustible gases can be used directly to fuel a gas turbine, or combusted in a power boiler.
Low pressure gasification requires an insulated environment, which is obtained through a refractory lined vessel. Refractory vessels of current design for use as gasifiers employ a stainless steel jacket and a fused-cast alumina liner. The alumina liner normally has a first inner layer of blocks comprising both alpha and beta alumina and a second outer layer of blocks comprising beta alumina. A small expansion allowance is provided between the outer layer of beta alumina blocks and the stainless steel jacket.
After vessels of this design are operated for a few months, it has been found that the refractory materials react with the soda in the liquor and expand to completely consume the normal expansion allowance provided between the refractory and the stainless steel jacket. At this point, the refractory layers begin to press against the inside of the stainless steel jacket. This situation causes early failure in the refractory materials themselves and plastic deformation of the stainless steel jacket. As a consequence, refractory linings of a conventional design have been unsatisfactory for use in a black liquor gasifier.
The inventors have found that such refractory materials are not only subject to thermal expansion as is known in the prior art, but also to chemical expansion. Sodium from the black liquor combines with alumina to form sodium aluminate. Sodium aluminate expands on the order of 130% relative to the original alumina. This causes both radial and vertical expansion in the refractory lining toward the metal shell of the vessel and also against the dome of the metal shell. Prior dome designs were torispherical in shape. Torispherical domes employ a layer of skew blocks at their base. Prior skew block layers were restrained from expansion by direct contact against the metal shell. This however resulted in over constraining and overstressing the refractory lining as well as the metal shell.
It is therefore desirable to provide controlled expansion in a radial direction of the skew blocks forming the bottom layer of a torispherical dome without stressing the refractory lining or the exterior metal shell. This problem is solved in accordance with the present invention by a refractory vessel having a metal shell and a specially designed restrainer for the skew block layer. The vessel comprises a generally cylindrically metal shell having an upper torispherical dome. A refractory liner has a cylindrical portion spaced inwardly from the shell and a torispherical portion spaced inwardly from the torispherical dome. A circular layer of skew blocks is located between the cylindrical portion and the torispherical portion. A metal band is positioned outwardly from the layer of skew blocks for restraining radial expansion thereof. The metal band has a predetermined strength such that the radially outward force exerted by the skew block due to the dead weight of the refractory lining in the dome loads the band in tension to a stress just less than the yield stress of the material. This predetermined strength is determined on the basis of the weight of the refractory lining and the geometry of the torispherical dome. When the refractory lining in the dome experiences expansion because of chemical reaction, the skew block migrates radially outward. Since the support band is already loaded almost to its yield stress, it elongates without much additional resistance and allows expansion of the lining. Additional support in the form of metal foam is provided between the band and the shell. This material provides the necessary resistance for the chemical expansion process.